SHIFT_DEFORMED Translating and Rotating a Model Back to its Undeformed Position

SHIFT_DEFORMED Translating and Rotating a Model Back to its Undeformed Position

Sometimes it is useful to be able to move a deformed structure back to its undeformed position, for example to measure knock-back (crush) following an impact. In many cases this will involve applying a rotation as well as a translation, and the FIX_NODE option described in FIX_NODE Fixing a node position despite displacements (which only applies a translation) will not be adequate.

Consider the following example: a ship hits a concrete caisson, punches a hole through it with some damage to its bow plates, and also pitches up as at tries to climb over the caisson. Measure the damage to the bow plates. The situation is shown in the figure below left, and in the figure below right the deformed and undeformed shapes of the ship are shown. Clearly the rotation the ship has undergone makes it hard to measure the deformation

By using SHIFT_DEFORMED to pick three nodes that are representative of the rigid body motion of the ship, translation and rotation can be applied to bring the deformed geometry back to overlay the undeformed, making measurement possible: see the figures left and right below). The SHIFT_DEFORMED panel is shown right. You can screen- PICK the nodes, or type them in directly. Once defined this mode can be switched on or off at will.

Choosing sensible nodes for SHIFT_DEFORMED .

The three nodes you choose form a right handed coordinate system, so they must not be colinear (or become colinear due to displacements), and the order of their definition is significant: see the figure below.

The local coordinate system [X',Y',Z'] is defined as shown here, and the inverse of this is applied to rotate the model back to its undeformed state.

The displacement of node 1 is subtracted from all nodes in the model to bring it back to the undeformed position.

( FIX_NODE applies this translation only, thus it is a subset of SHIFT_DEFORMED , which is why the two operations cannot be used at the same time.) You should try to choose three nodes whose relative position will not change too much as the model deforms, so that their motion is representative of the rigid body motion of the structure as a whole. And node 1, from which the rigid body translation if computed, is the most significant. For example in a frontal impact car crash analysis you should nodes at the back of the car. If your model has some rigid bodies then nodes on them would be ideal.

Note 1:	SHIFT_DEFORMED cannot be used at the same time as FIX_NODE , since the translations they apply would conflict.
Note 2:	SHIFT_DEFORMED can be used in conjunction with "explosion" vectors and magnified displacements. The order of application is given in Note 3 in Notes on explosion vectors.
Note 3:	SHIFT_DEFORMED uses the same three nodes as those in REFERENCE_GEOMETRY . The difference is that shifting the model simply changes the graphics displayed, whereas reference geometry changes the data values contoured and output. They can be used in conjunction or separately.

The SAVE/RETRIEVE button allows multiple local coordinate system definitions to be saved, retrieved and deleted from a 'csys.loc' file, written in the model directory. This means that local coordinate systems can be reloaded across different sessions of D3PLOT without having to recreate them.

Pressing the GET button brings up a list of available coordinate systems in both the 'csys.loc' file and any *DEFINE_COORDINATE_NODES definitions in the ZTF file.